This invention relates to lubrication of the valve mechanism of an internal combustion engine, and more particularly to a lubricating arrangement in the valve mechanism of an overhead camshaft engine.
The valve mechanism or valve motion of an internal combustion engine of the overhead camshaft (OHC) type is generally comprised of a plurality of camshaft holders mounted on the cylinder head, a camshaft rotatably supported by the camshaft holders, a pair of rocker shafts usually having a hollow tubular structure, secured to the camshaft holders, and at least one pair of rocker arms mounted on the rocker shafts for rocking motions thereabout and operable in response to the rotation of the camshaft for causing the intake valves and exhaust valves to be closed and opened alternately.
In such a valve mechanism, the rocker shafts are fitted in or through bearing holes in the camshaft holders, and are rigidly fastened to the cylinder head together with the camshaft holders by means of bolts extending through the rocker shafts and the camshaft holders.
Lubricating oil for lubricating the sliding parts of the valve mechanism is supplied from an oil pan provided in the engine, fed through oil passages formed in the cylinder block, the cylinder head and some of the camshaft holders, and guided into the hollow interiors of the rocker shafts. Then, the oil is delivered from the hollow interiors of the rocker shafts through small bores formed in the rocker shafts to lubricate the rocker arms and the camming surfaces of the camshaft, and also led to the intake valves and exhaust valves of the engine to lubricate same.
However, in order to ensure smooth fitting of the rocker shafts into or through the bearing holes of the camshaft holders, the clearances between the rocker shafts and the respective bearing holes have to be set at somewhat large values. Further, the rocker shafts have their peripheral surfaces held against the inner surfaces of the bearing holes of the camshaft holders by means of the fastening bolts. It is therefore difficult to machine and assemble the rocker shafts and the camshaft holders so as to adjust and hold the above clearances to and at desired fine values throughout the whole peripheries of the rocker shafts enough to keep sufficient oiltightness between the rocker shafts and the camshaft holders. As a consequence, there can easily occur leakage of oil through the gaps between the rocker shafts and the bearing holes, impeding positive feeding of a required amount of lubricating oil to various sliding parts of the valve mechanism such as the rocker arms.
To supply an increased amount of lubricating oil to the valve mechanism so as to make up for the leaked amount, an oil pump having a large capacity or discharge quantity is required, inviting an increased manufacturing cost. On the other hand, to reduce the leakage amount of oil per se, very tight tolerances are required in machining the rocker arms and the bearing portions of the camshaft holders and assembling these parts, also resulting in an increase in the manufacturing cost.
Besides the above disadvantages, the conventional lubricating arrangement has another disadvantage that the intake side rocker arms and the exhaust side rocker arms are supplied with lubricating oil through different lubricating systems, making the lubricating arrangement complicate in structure.